By Nana O 
Immune therapy has revolutionized cancer treatment, offering a powerful new arsenal against the disease by harnessing the body’s own defenses. However, a persistent challenge remains: many tumors possess an uncanny ability to camouflage themselves as healthy tissue, rendering them invisible to the immune system and resistant to even the most advanced immunotherapies. This inherent evasiveness has left countless patients with difficult-to-treat cancers facing limited options. Now, a groundbreaking discovery by researchers at the University of California, San Francisco (UCSF) is poised to change this landscape, revealing a previously unrecognized vulnerability in some of the most aggressive cancers, including deadly brain cancer (glioma).
The UCSF team has identified unique, "jumbled" proteins produced by certain cancers that act as distinctive molecular signatures, making them stand out from healthy cells. These newly recognized cancer-specific proteins, termed antigens, represent a significant leap forward in the development of potent immunotherapies capable of targeting and eradicating hard-to-treat tumors. This pivotal study, supported by grants from the National Institutes of Health, was published on February 19th in the prestigious scientific journal Nature.
The Genesis of Cancer’s Unique Antigens: A Splicing Symphony Gone Awry
The research illuminates a fascinating biological process: the creation of these novel antigens stems from errors in RNA splicing, a critical cellular mechanism that dictates how messenger RNA (mRNA) molecules—the blueprints for protein production—are assembled from smaller segments. In a diverse array of cancers, including those affecting the brain, prostate, liver, and colon, the study found that tumor cells splice together bits of RNA in unprecedented ways. This process generates entirely new RNA sequences, which in turn produce proteins, or antigens, that are absent in normal, healthy tissues.
Crucially, some of these newly formed RNAs give rise to antigens that are displayed on the surface of tumor cells. This surface presentation acts as a beacon, creating an accessible entry point for immunotherapies. The UCSF researchers demonstrated the therapeutic potential of this discovery by engineering immune T-cells to specifically recognize these cancer-specific antigens. In laboratory experiments, these engineered T-cells proved remarkably effective in destroying glioma cells, offering a tangible proof-of-concept for a new generation of cancer treatments.
This revelation holds the potential to dramatically expand the repertoire of targets available for immunotherapy. By identifying antigens derived from alternative RNA splicing, scientists can unlock a vast new frontier for therapeutic development, offering renewed hope and expanded treatment options for patients battling recalcitrant cancers.
A Paradigm Shift in Targeted Therapy
"We believe these initial antigens could be actionable in the near future, paving the way for novel therapies for glioma patients," stated Hideho Okada, MD, PhD, a professor of neurosurgery at UCSF and co-corresponding author of the study. "However, this is just the tip of the iceberg, and we are incredibly excited to delve deeper into the wealth of data we have generated to uncover many more such targets."
The current landscape of precision medicine primarily relies on two main strategies: drugs that inactivate specific mutant proteins driving cancer growth, or immunotherapies that direct immune cells to recognize cancer-related antigens. However, a significant hurdle is that many tumors either lack these characteristic mutant proteins or exhibit a limited range of antigens, leaving substantial portions of the tumor untouched.
"One of the primary reasons we suspect many glioma therapies falter is their tendency to target only a singular aspect of the tumor, allowing the rest to persist and evade treatment," explained Joe Costello, PhD, also a professor of neurosurgery at UCSF and co-corresponding author. "These newly identified antigens provide a crucial breakthrough, helping us overcome the formidable challenge of brain tumor heterogeneity."
The Art of "Fishing" for Antigens within the RNA Sea
The quest for novel cancer targets led Darwin Kwok, PhD, to meticulously investigate the intricacies of RNA splicing. This process can yield multiple protein variants from a single gene, and Kwok hypothesized that alterations in this mechanism could lead to the generation of unique cancer-specific antigens.
"While many current cancer therapies are built upon identifying unique DNA mutations within tumors, we suspected that tumors might also harbor altered RNA splicing patterns that could give rise to new, cancer-specific antigens," said Kwok, who is a recent PhD graduate from the Okada and Costello labs, currently a medical student at UCSF, and the lead author of the groundbreaking study.
Kwok embarked on an extensive analysis of RNA sequencing data from thousands of tumors, drawing from the comprehensive repository of The Cancer Genome Atlas, a program funded by the National Cancer Institute. His meticulous work focused on identifying uniquely spliced mRNA sequences that were consistently present across multiple biopsies within the same tumor and across different patients. The tumors examined in this phase originated from a range of common cancers, including prostate, liver, colon, stomach, kidney, and lung.
Complementing this broad analysis, Kwok collaborated with the UCSF Brain Tumor Center. This partnership allowed for the examination of mRNA from glioma samples donated by 51 UCSF patients. The research team obtained up to ten biopsies from each tumor, carefully cataloging the precise location of each biopsy within the tumor. This detailed approach further enabled the identification of unusual mRNA sequences.
A Thousand New Leads: Discovering a Reservoir of Cancer-Specific mRNAs
This comprehensive and multi-pronged investigation yielded an astonishing discovery: nearly 1,000 cancer-specific mRNA molecules that were not previously documented. These unique transcripts were found to be common across diverse tumor types, different cancer patients, and even across multiple cancer types, underscoring their potential as broadly applicable therapeutic targets. Crucially, none of these newly identified mRNAs were ever detected in healthy tissues, reinforcing their specificity to cancerous cells.
Predicting and Validating Prime Candidates for Immunotherapy
The journey from identifying potential mRNA targets to developing a viable immunotherapy is complex. Not every mRNA molecule translates into a protein, not all proteins are displayed on the cell surface as antigens, and not every antigen is recognized by the immune system. Therefore, the researchers employed sophisticated computational modeling to predict which of these thousands of newly identified mRNAs had the highest probability of progressing through this pathway to become effective immunotherapy targets.
This rigorous process narrowed down the initial pool to 32 promising antigen candidates, all arising from cancer’s aberrant RNA splicing. From this refined list, the team selected the top four for more in-depth experimental validation. These four antigens were chosen based on their structural similarities to other known antigens that effectively trigger an immune response.
From Laboratory Bench to Potential Lifesaving Therapy
The validation process began by engineering cells to display these four candidate antigens. Subsequently, immune cells, isolated from the blood of healthy donors, were introduced to these antigen-presenting cells. This critical experiment revealed the presence of specific receptors on these natural immune cells that could reliably detect the cancerous antigens. This finding was a significant milestone, confirming that the body’s immune system possessed the inherent capability to recognize these novel cancer signatures, a prerequisite for developing effective immunotherapies.
The probability of identifying these specific immune receptors in donated blood was remarkably low – described by Dr. Okada as akin to "one in five or 10 million." Yet, the team achieved a remarkable breakthrough: for two of the top four antigens, they successfully identified complementary immune receptors in two different healthy donors. This serendipitous discovery provided the essential components for constructing a targeted immunotherapy.
Engineering a New Weapon Against Cancer
With the identification of both the cancer antigens and the corresponding immune receptors, the researchers moved to the next stage: creating a functional immunotherapy. They programmed laboratory-derived T-cells to express these identified immune receptors. These specially engineered T-cells were then deployed against glioma cells in controlled laboratory environments. The results were striking: the engineered T-cells proved devastatingly effective, rapidly identifying and eliminating the glioma cells.
Broader Implications and Future Directions
The implications of this research extend far beyond brain cancer. The discovery that alternative RNA splicing generates a shared set of novel antigens across multiple cancer types suggests a potential for developing immunotherapies that could be effective against a wide range of malignancies. The UCSF team is now actively progressing this research by testing their approach in animal models of cancer, with the ultimate goal of rapidly translating these promising findings to clinical trials in human patients, should the preclinical studies prove successful.
Beyond the initial four validated antigens, the study identified a substantial list of 28 additional promising candidates that warrant further investigation. Moreover, the vast dataset generated by the researchers holds the potential for uncovering countless other cancer-specific antigens derived from alternative splicing events.
The precise biological reasons why so many different cancers consistently produce the same handful of aberrant proteins remain a subject of ongoing investigation. It is possible that these jumbled proteins are an incidental byproduct of the complex processes that drive cancer development. Regardless of the underlying cause, this discovery represents a significant new front in the ongoing battle against cancer.
"This advancement for cancer patients is the embodiment of collaborative innovation at UCSF, spanning from sophisticated computational modeling and cutting-edge laboratory validation to novel techniques in neurosurgery," emphasized Dr. Okada. "This is precisely the kind of breakthrough needed to tackle the most tenacious cancer cases and bring much-needed relief to our patients."
The research was supported by grants from the National Institutes of Health (NIH), including grants from the National Cancer Institute. The study’s publication in Nature underscores its significance and impact on the global scientific community. This discovery marks a pivotal moment in the pursuit of more effective and broadly applicable cancer immunotherapies, offering a renewed sense of optimism for patients facing the most challenging diagnoses.